In a magnetic disk drive, the positioning of the transducer head over data tracks in alignment with the track centerline is accomplished using a "servo loop" including a mechanical actuator with a feedback path utilizing servo patterns written at the time of manufacture on sector segments or surfaces of the disk pack reserved for servo information. The servo patterns, head parameters, disk parameters and detection methods are customized to the specific application to ensure a linear response across the track or portion of the track. Measurable characteristics of the functionality of the servo system are the linearity achieved, the range over which the linear region is maintained, and the repeatability with respect to the signal to noise ratio, defects, and variations in the head's and disk's physical dimensions and magnetic properties.
Recent advances in magnetic recording heads have incorporated a magneto resistive (MR) element to sense the flux from the media. The MR element is typically biased to provide linearity, polarity and maximum sensitivity for detecting transitions written on the disk. The effects of biasing the sensor in combination with the physical geometry of the sensor cause non-uniformity of the read sensitivity along the length of the sensor with greater amplitude sensitivity occurring near one end. The non-uniformity of the read sensitivity along the length of the sensor causes non-linearity and a reduction in the width of the linear region of the servo systems position error signal (PES) for an amplitude type detection method. The PES's linear region becomes progressively more curved and narrower as the sensor's response along the MR elements length becomes more asymmetrical.